This project will help to characterize the central nervous system mechanisms that regulate autonomic and respiratory compensation following hypovolemic hypotension and circulatory shock. Experiments will be conducted to test the hypothesis that caudal hindbrain serotonergic neurons activated by acidosis, stimulate 5-HT1A receptors to promote sympathetic-mediated venoconstriction of the splanchnic vascular bed during hypovolemia. It is further proposed that the preferential constriction of the venous vasculature induced by 5-HT1A receptor activation will produce less reperfusion injury during resuscitation from hypovolemic shock than clinically used vasoconstrictor agents which tend to constrict arterial vascular beds. Aim 1 will determine whether caudal hindbrain serotonin is critical for maintenance or recovery of sympathetic-mediated whole body venous tone and venous return following severe blood loss. Aim 2 will determine whether the acidemia associated with hypovolemia or respiratory and metabolic acidosis per se contribute to activation of caudal hindbrain serotonin neural activation and subsequent respiratory and autonomic responses. Further studies will assess with acidosis contributes to the maintenance of blood pressure through a preferential venoconstriction. Aim 3 will determine whether serotonin acts on 5-HT1A receptors to mediate compensatory responses to hypovolemia and whether this endogenous pathway can be exploited to produce a more favorable hemodynamic response during resuscitation from hypovolemic shock. These studies will rely heavily on a carefully developed in vivo rat and mouse models of hypotensive hemorrhage and hypovolemic shock. State of the art techniques for continuous monitoring of hemodynamic parameters, sympathetic nerve activity and central respiratory drive in unanesthetized animals will be used to assess cardiovascular parameters after pharmacological and molecular manipulation of serotonin and serotonin receptor levels. In addition, newly developed techniques for the recording of sympathetic activity in the unanesthetized mouse will enable use of genetically altered mice for investigation of the receptors involved in the compensatory responses to blood loss. Furthermore, novel molecular techniques to more acutely alter serotonin levels in discrete brain regions will be utilized to dissect regions important in the neural control of the circulatory responses to blood loss. Finally, pre-clinical, translational studies will address the potential utility of using 5-HT1A receptor agonists as adjuvants in resuscitation from circulatory shock. PUBLIC HEALTH RELEVANCE: Despite recent advances in emergency medicine, traumatic blood loss is currently one of the leading causes of death of individuals under 40 in the US. Patients typically succumb to severe blood loss either because of too little tissue perfusion or because of tissue injury incurred during the resuscitation process. Our studies will attempt to validate a new, promising therapy that may help patients recover from circulatory shock without further injuring tissue during the resuscitation process.